1. Field of the Invention
The present invention relates to rigid polyurethane foams, and more particularly to the preparation of such foams with alkyl alkanoates, preferably, methyl formate as the sole or primary blowing agent.
2. Description of the Prior Art
Rigid polyurethane foams are useful in a wide range of applications, including applications for insulation in refrigeration systems, structural applications and flotation applications, as in boats, buoys, docks and other flotation devices. In such applications, not only the density and closed cell content of the foam, but also the ability of the foam to retain its dimensional stability in adverse conditions are critical. In fact, in flotation applications, the United States government has specified rigorous test methods and standards for such characteristics of foams in relevant applications associated with the United States Coast Guard. See, for example, 33 CFR § 183.114 and the test methods set forth in ASTM D2126.
Conventionally, processes employing two components or three components have been used to prepare rigid foams. One component, generally referred to as component A (or A-side), comprises isocyanate. Frequently, component A also comprises a surfactant and a blowing agent. The second component, known generally as component B (or B-side), comprises any of various polyols, particularly polyether and/or polyester polyols. Component B may also comprise a surfactant, a catalyst package and a blowing agent, any or all of which may reside solely in component B. Alternatively, any or all of such ingredients may be introduced to the reaction mixture in a third stream or in both component B and a third stream. If an excess of isocyanate is employed, modified polyisocyanurate foams may be formed. The foams produced by these standard methods commonly have a density in the range of from about 1 lb./ft.3 to about 4 lb./ft.3, have a closed cell content on the order of about 85% and have low friability characteristics.
Ordinarily, the polyols are poly-functional; that is, the polyol molecule has two or more branches, which provide linking sites for cross-polymerization. Historically, the blowing agent is or at least includes one or more chlorofluorocarbon (CFC), often in combination with water. However, CFCs have been the subject of environmental concern in view of the deleterious effects that they have been reported to have on the earth's ozone layer. Therefore, it is desirable to avoid use of CFCs and the industry for many years has been searching for viable alternative blowing agents suitable for preparation of rigid polyurethane foam.
Certain processes that employ HCFCs in place of CFCs have been reported. For example, U.S. Pat. Nos. 5,032,623, 5,194,175 and 5,274,007 describe a process by which an HCFC, monochlorodifluoromethane (CHClF2), can replace traditional CFCs in the preparation of rigid foams. However, even HCFCs, while markedly better than CFCs in terms of deleterious environmental effects, still are associated with environmental risks and so it is optimal if even their use is avoided.
Attempts at the use of alkyl alkanoates, including methyl formate, as an alternative blowing agent have been reported, but those attempts have been unsuccessful in that the foams that have been produced with methyl formate as the primary or sole (other than, perhaps, water) blowing agent are unsatisfactory. In particular, it has been found that such foams have a foam shrinkage of more than 10% and so demonstrate poor dimensional stability. See, for example, U.S. Pat. No. 5,283,003 to Chen. Thus, use of methyl formate as a blowing agent has been reported to produce a dimensionally stable foam only when used in combination with a substantial amount of one or more additional blowing agents such as organic or even hydrocarbon or traditional CFC or HFC types of blowing agents. According to the U.S. Pat. No. 5,283,003 to Chen, methylene chloride and/or five-carbon hydrocarbons (e.g., n-pentane, isopentane, and/or cyclopentane) blowing agents must make up at least about 20% by weight of the total blowing agent combination. U.S. Pat. No. 5,883,146 to Tucker describes the use of formic acid (or a salt thereof) as a blowing agent, but only in combination with a C1 to C4 hydrofluorocarbon. Although the patent to Tucker does not define the necessary relative proportions of the formic acid (or salt thereof) and the hydrofluorocarbon, the exemplification show that the hydrofluorocarbon must make up at least about half the blowing agent combination by weight.
Accordingly, the rigid foam industry is still searching for methods and compositions that would enable methyl formate or a related compound to be used as a blowing agent that can produce a rigid foam that meets the standards for such foams, but without the need for other organic blowing agents. Such standards include not only those relating to density and closed cell content, but also those relating to dimensional stability (resistance to shrinkage) as well.